FIGS. 23A and 23B show a configuration of a conventional charged particle accelerator described in Patent Document 1 listed below. This charged particle accelerator is a cyclotron, which is a representative example of a charged particle accelerator with a spiral trajectory. In FIGS. 23A and 23B, 70 denotes a magnet, 71 and 72 denote accelerating electrodes, and 73 denotes a radio-frequency power supply that supplies an accelerating radio-frequency voltage to the accelerating electrodes 71 and 72. Furthermore, 74 denotes a charged particle that is accelerated by the accelerating electrodes 71 and 72.
In the cyclotron, a period Tp of revolution of the charged particle 74 satisfies the relationship Tp=2πm/eB, where n denotes the ratio of the circle's circumference to its diameter, m denotes the mass of the charged particle 74, e denotes the electric charge of the charged particle 74, and B denotes the magnetic flux density on a particle trajectory attributed to the magnet 70. Therefore, provided that m/eB is constant, the period of revolution of the charged particle 74 is constant regardless of the radius of revolution. For example, when a period Trf of the accelerating radio frequency of the radio-frequency power supply 73 satisfies the relationship Trf=Tp/2, the charged particle 74 is constantly accelerated in an electrode gap between the accelerating electrodes 71 and 72, and therefore can be accelerated to a high energy.
When the speed of the charged particle 74 approaches the speed of light, the value of the mass m of the charged particle 74 increases due to relativistic effects. As a result, in the cyclotron shown in FIGS. 23A and 23B, the isochronous properties cannot be ensured when the accelerating energy of the charged particle 74 increases to the extent that its speed approaches the speed of light, thus making it impossible to continue further acceleration. As a countermeasure against the above issue, it has been suggested to, for instance, change the magnetic flux density or the period of the accelerating radio frequency in accordance with an increase in the accelerating energy.